1. Field of the Invention
The present invention relates to a circuit configuration for use with a switched power supply of the forward converter type.
2. Description of the Related Art
xe2x80x9cAttention has been focused on the input current total harmonic distortion (THD) due to the increased use of nonlinear loads that tend to degrade AC line quality. THD standards, such as the IEC-1000-3-2 promulgated by the international Electrochemical Commission, have been adopted. Due to requirements associated with cost effectiveness, there are numerous references in the prior art directed to single power stage AC-DC converters which employ such nonlinear loads. The power supply used with these converters usually requires multiple output DC voltages and fast output regulation. In addition, a high frequency power transformer is used to step up/down the output DC voltage. The transformer presents several drawbacks. First, a higher voltage stress occurs across the power switch due to the transformer volts-second reset. Second, the leakage inductance of the transformer, usually causes a voltage spike across the power switch when it turns off. To minimize or reduce the voltage spike, a snubber circuit, such as a R-C-D snubber, is sometimes used to absorb this voltage spike. This is not an optimum solution, however, in that the leakage energy of the transformer is dissipated in the snubber circuit making the circuit inefficient.xe2x80x9d
One prior art single power stage AC-DC forward converter is shown in FIG. 1, where Lk is the leakage inductance of the transformer T, Ds, is the body diode and Cds is an internal capacitance of switch S. The inductor Lin is a boost inductor and is used to shape the input current waveform to achieve a high power factor with low input current harmonics. Winding N3, coupled with windings N1 and N2, is used to reset the transformer T.
FIGS. 2a -2b illustrate typical switching waveforms associated with the circuit of FIG. 1. The voltage on DC bus capacitor Cdc, i.e., Vdc, is considered to be constant over successive switching cycles as a consequence of the large capacitance value selected for Cdc in comparison with the switching period. During the time switch S is on, time (t1-t2), voltage Vdc is applied to primary winding N1 and leakage inductor Lk. Therefore, the voltage Vdc on capacitor Cdc is discharged to the load Ro via transformer coupling N1-N2. The difference between the reflected voltage across winding N2 and the voltage across the load Ro, i.e., Vo, is applied to the choke inductor Lf as Vlf for the period (t1-t2). As a result, an increased current produced in Lf is delivered to the output load Ro. Also, during time interval (t1-t2), a circuit path is formed from the AC voltage source Vin, through inductor Lin, diode D5, switch S and back to ground. Energy is stored in inductor Lin during this time.
Considering the time during which switch S is off, i.e., (t2-t4). Diode D5 is nonconducting. The current through Lin therefore flows through diode D4 charging capacitor Cdc. It is noted, however, this charging current is inconsequential given the large capacitance value of Cdc and the relatively short charging interval. A magnetizing current in winding N1, which previously traveled through switch S during the time switch S was on, is now transferred (magnetically coupled) to winding N3 causing diode D1 to transition to a conducting state. A voltage developed across winding N3 by virtue of the coupled magnetizing current ser-cs as additional charging source for capacitor Cdc. During this time interval (t2-t4), the winding current in N3 is effectively reset. A negative voltage which is developed across N2 in the secondary winding causes diode D2 to become nonconducting. As such, energy stored in Lf during the time (t1-t2) is released through the load Ro for the time (t2-t4).
A drawback associated with the circuit of FIG. 1 is that at time t2, the point at which switch S turns off, the energy which was stored in leakage inductor L during the time (t1-t2), i.e., the time switch S was ON, is released to charge internal switch capacitor Cds thereby generating an undesirable voltage spike as shown in FIG. 2b. After the leakage energy is released, the voltage across switch S is equal to the sum of the DC bus voltage plus the tranformers reset voltage. A negative consequence of the higher voltage across switch S is that a higher rated switch S is required, especially for a higher input line voltage, e.g., 277 AC. An associated drawback of using higher voltage rated switches is their increased cost.
A number of circuit topologies have been proposed in the prior art to eliminate the large overvoltages which occur across switching devices at turn off. One method for minimizing the large overvoltages is the use of an R-C-D snubber network. In general, the function of a snubber circuit is to reduce the electrical stresses placed on a device during switching by a power electronics converter to levels that are within the electrical ratings of the device. In the present case, a snubber network is employed to limit voltages applied to a switch during turn-off transients.
FIG. 3 illustrates the prior art circuit of FIG. 1 with the addition of an R-C-D snubber network. While the snubber network minimizes or reduces the occurrence of voltage spikes, it does so at the expense of reducing the conversion efficiency of the circuit. In addition, snubbers introduce additional complexity and cost to the basic converter circuit. As such, it is a non-optimal solution.
Thus, it is desirable to provide a converter design that limits the maximum voltage stress on the power switch and recycles the leakage energy of the transformer such that the circuits efficiency is enhanced.
According to the invention, a single power stage AC/DC forward converter with power switch voltage clamping function includes: a switch S; Lk1 and Lk2 representing leakage inductances associated with the primary windings of transformer T, two series clamping and recovery capacitors Cd1 and Cd2; a transformer T including primary windings N1 and N2 having the same number of turns; a boost inductor Lin; a filter Lf; a diode D connected in parallel with the switch S and in series with Cd1 and Cd2.
In one embodiment, the DC bus capacitors Cd1, and Cd2, are connected directly to the leakage inductors Lk1and Lk2, respectively. In this embodiment, during intervals between conduction by switch S, the capacitors recover leakage energy from the leakage inductors Lk1, and Lk2, and during intervals of conduction by switch S, the recovered leakage energy stored in capacitors Cd1 and Cd2, is provided to the load via transformer T.
A main advantage provided by the circuit of the present invention is the prevention or substantial elimination of voltage spikes which would otherwise occur at each switch transition to the OFF state. Voltage spikes are eliminated in two ways: (1) by configuring the DC bus capacitors Cd1, and Cd2 to be in parallel with the switch S and, (2) by selecting the capacitance values of capacitors Cd1 and Cd2 to be sufficiently large to clamp the voltage across switch S to a value equal to the DC bus voltage.
A further advantage of the circuit of the present invention is that by recovering the leakage energy in each switching cycle, as opposed to dissipating it in accordance with prior art approaches, the overall circuit efficiency (i.e., power out/power in) is enhanced. An additional advantage of capturing the leakage current is that the voltage rating of switch S is significantly reduced thereby reducing its cost.
Accordingly, it is an object of the invention to provide an AC/DC forward converter in which the voltage across the main switch S is clamped to the DC bus voltage, thereby preventing the occurrence of undesirable voltage spikes.
It is another object of the invention to provide an AC/DC forward converter in which the leakage energy of the transformer is substantially recovered to improve circuit efficiency.
It is yet another object of the invention to meet the objectives stated above with a converter in which a minimum number of components are used to reduce costs.
Still other objects and advantages of the invention, will, in part, be obvious and will, in part, be apparent from the specification.